The present invention relates to polyol polymers cured with multi-isocyanate cross-linking agents and more particularly to such a system which is curable in the presence of vaporous tertiary amine catalyst wherein no curing chamber is required.
Vapor permeation curable coatings are a class of coatings formulated from aromatic-hydroxyl functional polymers and multi-isocyanate cross-linking agents wherein an applied film thereof is cured by exposure to a vaporous tertiary amine catalyst. In order to contain and handle the vaporous tertiary amine catalyst economically and safely, curing chambers, eg. U.S. Pat. Nos. 3,851,402 and 3,931,684, were developed. Such curing chambers typically are substantially empty rectangular boxes through which a conveyor bearing the coated substrate passes. Provision is made for entrance and exit of vaporous tertiary amine, normally borne by an inert gas carrier such as nitrogen or carbon dioxide, for example, and means at the inlet and the outlet of the chamber to enhance containment of the vaporous tertiary amine catalyst within the chamber. The inlet and outlet contain means further restrict the entrance of oxygen into the chamber because oxygen can create an explosive condition with the vaporous tertiary amine catalyst. Cure of such coatings is so rapid that no external source of heat is required. An apparent drawback of such curing chambers is the capital investment required and the amount of space which such curing occupy in the plant. For example, such chambers can range up to 40 or 50 feet or longer in order to ensure sufficient contact time between the curable coated substrate and vaporous amine atmosphere. Also, chambers must be specially designed in order to handle large parts, eg. automotive parts, for curing. While such chambers can be engineered, extra expense in their manufacture, operation, and maintenance is required.
One proffered alternative to such vapor permeation curing chambers is the use of dual component spraying systems. For example, commercial spray equipment includes spray guns adapted to spray liquid coating compositions which must be separated from a source of catalyst. Such systems normally employ a mixing head or manifold which immediately precedes the spray tip. Such delayed mixing in the spray process minimizes the opportunity for the catalyst and coating composition to prematurely gel. Excellent discussions of such dual component or catalyst spraying can be found in the Finishing Handbook, Chapter 4, p.227 (1973) and Chapter 4, pp 195-230, especially page 223 (1980); Product Finishing, Volume 28, No. 6 (June, 1975) and pp 48-55 (March, 1978). The liquid catalyst, optionally dispersed in solvent, is shown to be delivered under pressure of air to the spray gun as is the liquid coating composition.
Another dual spray method involves the simultaneous spraying from two spray nozzles of the liquid coating composition and catalyst component separately as proposed in U.S. Pat. No. 3,960,644. U.S. Pat. No. 3,049,439 provides a design for a spray gun wherein the accelerator or catalyst and resin are premixed within the spray gun in an atomizing chamber immediately prior to exiting from the gun. U.S. Pat. No. 3,535,151 describes the selective addition of water and a thickener to a substantially dry liquid polyester resin in the spray gun as the polyester resin is being sprayed. U.S. Pat. No. 3,893,621 proposes a multi-nozzled spray gun which discharges an airless spray of liquid promoted resin from a first pair of nozzles and low pressure air atomized liquid catalyst from a second pair of nozzles wherein each atomized stream is mixed by intersection of each atomized stream exiting the spray gun immediately prior to deposition onto a substrate. U.S. Pat. No. 4,322,460 proposes to utilize a conventional two-component spray nozzel with mixing of a polyester resin and a benzoyl peroxide catalyst dissolved in cyclohexanone in the mixing head of the spray nozzle. U.S. Pat. No. 3,249,304 proposes to eliminate the possible polymerization of the catalyzed liquid resin within the mixing head of the spray gun during periods when the gun is not being used by providing a solvent wash line which flushes the mixing chamber during periods when the coating composition and catalyst are not fed into the spray gun. U.S. Pat. No. 3,179,341 provides yet another design for the mixing head within the spray gun for multi-component systems which include a resin and catalyst therefor. U.S. Pat. No. 1,841,076 intersects a spray of coagulable rubber and coagulant vapor wherein the coagulable rubber streams are created from two separate spray nozzles. Similarly, U.S. Pat. No. 2,249,205 takes two separate spray guns and intersects a stream of removable latex and atomized fluid coagulant which intermixed streams then are applied to a substrate. U.S. Pat. No. 4,195,148 (and U.S. Pat. No. 4,234,445) utilizes a conventional internally mixed spray gun as described above for spraying a mixture of a polyurethane prepolymer and an isocyanate curative therefor.
As the Examples will demonstrate, the obvious choice of a conventional two-component mixing spray nozzle for use with vapor permeation curable coatings was unsatisfactory because the catalyzed liquid coating composition gelled so rapidly that the spray gun became sufficiently plugged and spraying ceased. Thus, a new method for utilizing spray application for vapor permeation curable coatings was required. The present invention is directed to such a novel spray method.